Scanning electron microscope

ABSTRACT

A technique executes autofocus adjustment stably even when a plurality of patterns or foreign matter capable of being imaged only by a specific detector are included independently. Such an image as a concavo-convex image having a weak contrast can be picked up. The technique can automatically focus such an image even when it is difficult to find a focus position in the image. A scanning electron microscope includes a plurality of detectors for detecting secondary signals from a specimen when irradiated with an electron beam, and a calculation unit for combining the signals obtained from the detectors. At least two of the detectors are provided to be symmetric with respect to the electron beam. The focus of the electron beam is adjusted based on the signals of the detectors or on a signal corresponding to a combination of the signals.

BACKGROUND OF THE INVENTION

The present invention relates to scanning electron microscopes and moreparticularly, to a scanning electron microscope which performs autofocusadjustment on a plurality of types of images having different features.

For increasing the yield of semiconductor devices, an inspection step isindispensable. For increasing a production efficiency by locating acause of yield reduction, an electron microscope for inspecting andidentifying an abnormality in a wafer is indispensable. For efficientinspection, it is indispensable to speed up the inspection and thus itis desirable to inspect a number of points in a short time.

One of the elements to be considered for the inspection speed-up is anautofocus adjustment function of adjusting the focus of an image in anelectron microscope, which function is indispensable for picking up aclear image. One of such functions is disclosed, for example, inJP-A-2005-332593 as a method of achieving accurate autofocus. In theautofocus adjustment function disclosed in JP-A-2005-332593, a pluralityof images are acquired by changing a focus, high frequency responses forthese images are calculated, and a focus position corresponding to thebiggest one of the responses is calculated for automatic focusadjustment.

SUMMARY OF THE INVENTION

In the case of a semiconductor wafer, various sorts of foreign matter ordefects causing faulty device operation may be present in the wafer. Thewafer may have, for example, a relatively observable pattern edge, suchthat substantially no height of stain-like foreign matter of a chemicalshould still remain after chemical removal. An example of a defect thenis a very low level of raised and recessed portions on the surface suchas a scratch caused by polishing operation. As another example, a lowlevel of hard-to-observe and hemi-spherical raised and recessed portionsmay be caused when a thin film is formed on the wafer, because the filmis formed on the wafer already including such an irregularity. Some ofthe aforementioned systems, including a microscope for inspecting suchforeign matter or defects, can acquire a plurality of images havingdifferent physical characteristics using a plurality of detectors. Thesystem picks up an image according to the feature of detected foreignmatter or defects. When a plurality of sorts of foreign matter ordefects capable of being detected only in the image of each detector arepresent, however, there occurs such a situation that the aforementionedautofocus adjustment function cannot cope with it.

Consider a case which follows as an example. FIG. 1 schematically showsa cross-sectional view of an electron microscope, with only constituentelements necessary for explanation shown therein. In the drawing,reference numerals 122 and 123 denote detectors which play a role ofdetecting secondary electrons or reflected electrons emitted from awafer when irradiated with an electron beam emitted from an electron gun101. An image of signals detected by the detectors can be confirmed byestablishing synchronism between the scanning of the electron beamissued from the electron gun and a scan signal of a monitor (display)117.

In general, a secondary electron emitted from a specimen when irradiatedwith an electron beam is defined as having an energy of 50 eV or less.An electron emission efficiency varies from specimen to specimen, andsuch secondary electrons appear on a display screen in the form of blackand white grayscale. Such a corner as an edge of a pattern is featuredby having the amount of secondary electrons generated larger than thatat the other area even for the same substance. Meanwhile, a reflectedelectron is defined as having an energy of 50 eV or more, has a propertyof reflecting the shape of the irradiated material, and the yield ischanged with the installation direction of the detector. For example, asto a shallow hole area in a specimen, a part of the hole which faces thedetector appears as a bright part; whereas, a part of the hole which isopposed to the detector appears as a dark part. For example, utilizingthe above property, when the detectors are located to be opposed to eachother, a pair of shadow images having different shades can be acquired.Since the behavior of secondary or reflected electrons emitted from thespecimen varies with physical conditions or with an electric fieldbetween the specimen and the detector, it is difficult for each detectorto distinguish between the secondary and reflected electrons and detectthem. However, a certain degree of yield can be attained by controllingthe locations of the detectors or the electric field.

In the prior art, focus adjustment has been carried out by using eitheran image generated from a secondary signal detected by a secondaryelectron detector or an image generated from a secondary signal detectedby a reflected electron detector. However, in the prior art, it isdifficult, in all cases, to achieve suitable automatic adjustment. Forexample, consider situations where a specimen has substantially nothickness of stain-like foreign matter or where a specimen hasconcavities and convexities which have few difference in height, in anarea other than a pattern. In the former case, the stain-like foreignmatter clearly appears in a secondary electron image, but does notsubstantially appear in a reflected electron image because the stain hassubstantially no thickness. Meanwhile, the concavities and convexitiesclearly appear in the reflected electron image, but do not clearlyappear in the secondary electron image, because the concavities andconvexities include no difference of a material and no edge. For thisreason, when a defect is detected on the basis of an image based on thedetector for detecting secondary electrons (hereinafter referred to as asecondary electron image), the use of an image based on the detectorsfor detecting reflected electrons (hereinafter referred to as areflected electron image) and including a defect may, in some cases,result in the system not correctly performing its automatic adjustmentwhich results in an out of focus image. The same holds true for theopposite case. When the specimen includes a circuit pattern or the likearound the foreign matter, adjustment can be carried out based on itsimage. When the specimen includes no such circuit pattern, however, theadjustment cannot be carried out based on the image.

As has been mentioned above, the prior art has a problem that, when aspecimen includes such foreign matter and a pattern capable of beingpicked up only by a specific detector respectively and independently,accurate autofocus adjustment cannot be achieved based on a single imageby a detector.

It is therefore an object of the present invention to provide atechnique for stably achieving autofocus adjustment even when a specimenincludes a plurality of patterns or foreign matter appeared in an imageonly by a specific detector respectively and independently. An object ofthe present invention is also to provide a technique for achievingautomatic adjustment even when it is hard to find a focus position insuch an image having a weak contrast as a concavo-convex image, thoughthe image can be picked up.

Some or all of the above objects may be attained for example by ascanning electron microscope which includes a plurality of detectors fordetecting secondary signals from a specimen when irradiated with anelectron beam emitted from an electron gun, and a calculation unit forcombining the signals obtained from the detectors. In the exemplaryscanning electron microscope, at least two of the detectors are arrangedto be axially symmetric with respect to the electron beam, and a focalpoint of the electron beam is adjusted on the basis of the respectivesignals of the detectors or the combined signal thereof.

In accordance with another aspect of the present invention, there isprovided a scanning electron microscope which includes an electron lensfor converging an electron beam emitted from an electron gun onto aspecimen, a pair of first and second detectors and a third detector fordetecting secondary signals from secondary electrons emitted from thespecimen when irradiated with the electron beam, the first, second andthird detectors being provided to be spaced from each other by apredetermined spacing; a storage unit for generating and storing first,second and third pieces of secondary signal data from the secondarysignals detected by the first, second and third detectors for focuses ofthe electron beam to the specimen changed by a plurality of times; adata calculation unit for combining first, second and third pieces ofsecondary signal data for the respective focuses; a focus adjustmentunit for calculating an evaluation value on the basis of an intensitysignal obtained by subjecting the combined secondary signal data for thefocuses to predetermined data conversion and adjusting the focus of theelectron beam to the specimen using a focus calculated based on theevaluation values of the focuses.

In accordance with a further aspect of the present invention, there isprovided a scanning electron microscope which includes an electron lensfor converging an electron beam emitted from an electron source onto aspecimen; a pair of first and second detectors and a third detector fordetecting secondary signals obtained from the specimen when irradiatedwith the electron beam, the first, second and third detectors beingprovided to be spaced in each pair from each other by a predeterminedspacing; a storage unit for generating and storing first, second andthird pieces of secondary signal data from the secondary signalsdetected by the first, second and third detectors for respective focusesof the electron beam to the specimen changed by a plurality of times; adata calculation unit for calculating the first, second and third piecesof secondary signal data at the respective focuses; a detector selectionunit for selecting at least any one of the first, second and thirddetectors; and a focus adjustment unit for generating secondary signaldata from the detector selected by the detector selection unit bychanging the focus of the electron beam, finding an evaluation value ofthe secondary signal data for the focuses changed based on intensitysignals obtained by subjecting the generated secondary signal data topredetermined data conversion, determining a fitting functioncorresponding to the evaluation value, calculating an error between theevaluation value and the fitting function, adjusting the focus of theelectron beam to the specimen using a focus giving a peak value to thefitting function when the error is in a predetermined reference range,selecting the detector different from the detector selected by thedetector selection unit when the error is out of the predeterminedreference range, calculating an error between the evaluation value andthe fitting function with respect to the different detector, andadjusting the focus of the electron beam to the specimen using focuseshaving the errors lying in the predetermined reference range.

For the purpose of establishing stable autofocus adjustment at alltimes, the present invention can create a combined image using all orsome of images picked up by a plurality of detectors and can attain theautofocus adjustment. Even when a specimen includes such a pattern as tobe observable only by a specific detector or includes a plurality offoreign matter independently or even when an image has such a lowcontrast as to make it difficult to achieve focus adjustment, thepresent invention can stably perform the autofocus adjustment.

Furthermore, when an image that is capable of being picked up only by aspecific detector is included in the images, an electron microscopehaving a plurality of detectors can achieve stable autofocus adjustmentby picking up images by each detector while changing a focus position(Z), creating a combined image using part or some of the images, andevaluating the combined image. Thus even when it is difficult to achieveautofocus adjustment with use of a single image, the present inventioncan detect a just-focused position.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an arrangement of a scanning electron microscope to whichthe present invention is applied;

FIG. 2 is a flow chart for explaining the processing contents of anembodiment 1 of the present invention;

FIG. 3 is a diagram for explaining image acquisition and a calculationtarget in the autofocus function of the embodiment 1 of the presentinvention;

FIG. 4 shows a normal profile graph having evaluation values in theembodiment 1 of the present invention;

FIG. 5 shows an abnormal profile graph having evaluation values in theembodiment 1 of the present invention;

FIG. 6 shows a GUI (Graphical User Interface) used to set calculationcontents in an embodiment 2 of the present invention;

FIG. 7 shows a flow chart for explaining the processing contents of theembodiment 2 of the present invention;

FIG. 8 shows a flow chart for explaining the processing contents of anembodiment 3 of the present invention;

FIG. 9 is a diagram for explaining parameters used in a calculationmethod selection process in the embodiment 3 of the present invention;

FIG. 10 is a flow chart for explaining the processing contents of anembodiment 4 of the present invention;

FIG. 11 shows a GUI used to set the operational contents of an autofocusfunction in embodiments 4, 5 or 6 of the present invention;

FIG. 12 is a flow chart for explaining the processing contents of theembodiment 5 of the present invention;

FIG. 13 is a flow chart for explaining the processing contents of theembodiment 6 of the present invention;

FIG. 14 is a flow chart for complementarily explaining the operation ofa step 1304 in FIG. 13 in the embodiment 6;

FIG. 15 is a display screen on which autofocus adjusting contents isdisplayed in the embodiment 6 of the present invention; and

FIG. 16 is a display screen on which autofocus adjusting contents isdisplayed in the embodiment 6 of the present invention.

DESCRIPTION OF THE INVENTION

A scanning electron microscope shown in FIG. 1 includes an electron gun101, a lens 102, a deflector 103, an objective lens 104, a specimencarrier base 106, a lens controller 110, a deflection controller 111, anobjective lens controller 112, an analog/digital converter 113, anaddress controller 114, an image memory 115, a control unit 116, adisplay 117, a computer 118, an image processor 119, a keyboard 120, amouse 121, a secondary particle detector 122, a pair of reflectedelectron detectors 123, and an input unit to a movement stage 124. Themicroscope also includes a specimen 105, an electron beam 107, asecondary electron 108, and a reflected electron 109. In the drawing, acolumn for keeping a vacuum in the microscope is omitted. The pair ofreflected electron detectors 123 are located at positions opposed toeach other with respect to a straight line to pick up a dual shadowimage, but such a location is not limited to such an example as shown inthe drawing.

The electron beam 107 emitted from the electron gun 101 is converged bythe lens 102, two-dimensionally scanned and deflected by the deflector103, converged by the objective lens 104, and then applied to thespecimen 105. The application of the electron beam 107 to the specimen105 causes the secondary electrons 108 or the reflected electrons 109 tobe emitted from the specimen according to the shape or the material ofthe specimen. These secondary electrons 108 and reflected electrons 109are detected by the detectors 122 or 123, amplified, and then convertedto digital values by the analog/digital converter 113. Signals from thepair of reflected electron detectors 123 are used to form L and R imagesas reflected electron images; while a signal from the detector 122 isused to form an S image as a secondary electron image. Data on theconverted digital values is stored in the image memory 115. As anaddress in the image memory 115 at this time, the address controller 114generates an address synchronized with a scan signal of the electronbeam. The image memory 115 transmits, as necessary, image data about thestored SEM image to the computer 118. The computer 118 performs variousoperations including calculating a focus evaluation value from theimage, fitting the function to the evaluation value, calculating a peakon the fitting function, transmitting a focus adjustment signal to theobjective lens controller 112, and controlling the objective lens forfocus adjustment. The objective lens controller 112 is also used forfocus adjustment when an image is picked up during adjustment of thefocus position in autofocusing operation. The image processor 119 readsout image data from the image memory 115 and executes its operation. Theimage processor may have a plurality of functions, but it is required tohave at least a function of performing adding or subtracting operationon an image and also a function of performing multiplying operation by aconstant value.

The specimen 105 to be observed by the scanning electron microscope isheld on the specimen carrier base 106. The movement stage 124 cantwo-dimensionally move the specimen base under control of a controlsignal from the control unit 116, whereby the scanning position of theelectron beam 107 to the specimen 105 can be changed.

Embodiment 1

FIG. 2 is a flow chart showing a focus adjusting procedure. In FIG. 2,with such an arrangement as mentioned above, focus adjustment isexecuted according to a procedure of steps 201 to 209. In the step 202,first of all, images by each detector are picked up at focus positionsstepwise shifted by the objective lens controller 112. In this step, aset of images with focus states gradually varying with focus movementcan be obtained.

In the step 203, next, the set of images are combined. The imagesacquired in the step 202 are calculated according to apreviously-registered equation by the image processor 119.

FIG. 3 shows a diagram for explaining the processing contents of thestep 203. Images are picked up with varying focuses for each detector inthe step 202. Groups of images 301, 302, and 303 are images obtainedfrom the detectors of secondary electron, reflected electron (L), andreflected electron (R) respectively. In this connection, the reflectedelectron (L) indicates an electron detected by one of the detectors 123in FIG. 1, and the reflected electron (R) indicates an electron detectedby the other reflected electron detector 123. A vertical directionindicates a height direction, and the focus state is gradually changedwith the movement of the focus. Such images picked up with the samefocus as images 304, 305, and 306 are used to create a combined image308 according to an equation (1) which follows.α·SE+β·L+γ·R  equation (1)

In this equation, SE, L and R denote addition elements of a secondaryelectron image, reflected electron L and R images respectively; and α, βand γ denote coefficients of SE, L and R respectively.

In this way, the groups of images 301, 302, and 303 are calculated tocreate a group of images 307. According to the above equation (1), animage corresponding to an addition of a difference between the reflectedelectron images to the secondary electron image at a specific rate isoutput, and a secondary electron image having an enhanced contrast canbe obtained. The above equation (1) is not limited to this contents andthe addition elements, the coefficients and so on are not limited to theabove example.

In the step 203, next, an evaluation value for evaluating the focusstate is calculated. The evaluation value is calculated on the basis ofa differential value of the image. A pattern edge or the like can beeasily observed. However, such an edge at the contour or the like of afocused image has a grayscale level change larger than an edge at thecontour in the blurred image. From this viewpoint, a sum of pixel valuesafter the image is differentiated is calculated and the sum is used asthe evaluation value. Maximum one of the evaluation values is consideredto be equal to a focused position. More specifically, image data ispassed through such a differentiation filter as Laplacian to calculate asum of pixel values as the filtered result.

With respect to such evaluation values, when such a graph havingevaluation values as its vertical width and having a focus position (Z)as its horizontal width as shown in FIG. 5 is considered, approximationcan be realized with a curve having the maximum evaluation value at thefocused position. As the then curve (fitting function), a paraboliccurve, a polynomial curve or the like is used. To this end, when theabove fitting function is applied to the evaluation value data, afocused position can be found from its peak position.

At this time, with respect to the calculated fitting function, when anoffset or deviation from original data is evaluated and an error betweenthe calculated fitting function and the original data is not large, thedata can be determined as data usable for calculation of the focusedposition, and the amount of control of the objective lens to adjust thefocus at the focused position can be calculated by finding the peakposition of the fitting function.

When the error is large, this means that the deviation from the fittingfunction is large. Thus it becomes, in some cases, impossible tocalculate a correct peak position. For example, consider cases where onedetector has a sufficient image S/N ratio and the other detector has aninsufficient image S/N ratio at the same place. In the former case, anevaluation value graph is as shown in FIG. 4. In the graph, a horizontalaxis denote focus position (Z), a vertical axis denotes evaluationvalue, a black dot 401 indicates a plot of an evaluation value from eachpicture. When the S/N ratio is sufficient, the plots of the evaluationvalues follow a constant fitting function 402. Thus when a peak position403 on the fitting function is found, a just-focused position can becorrectly calculated. In the latter case where the selection of thedetectors is not suitable and the detector has an insufficient S/N ratiodue to a pattern or the like, however, the number of noise components isincreased. Thus a white dot 501 has an error larger than the black dot401 when the evaluation value is normal. Even the fitting under thiscondition results in that the dot fails to indicate the peak position403 of the fitting function 402 indicative of the correctly focusedposition and that an erroneous peak position 503 is determined as apeak.

In this way, fitting is carried out using a prescribed fitting function,and an error between the function and the evaluation value is used as acertain rule of thumb for determining whether or not the selected imageis suitable for autofocus adjustment. When the data has an error notlarger than a constant value, the system goes to the step 206 and afocus position is calculated with the same data. When the peak isinsufficient, the system proceeds to the step 208, where no focuscalculation is carried out and the states of constituent elementsincluding the objective lens are returned to their initial state. Thesystem then goes to the step 209, thus terminating the autofocusadjustment process.

The system calculates a peak position in the step 206, and then proceedsto the step 207, where a signal is sent to objective lens controller 112so that the focus position of the lens is adjusted at the focusedposition for focus adjustment. Since the peak position of the graph ofthe evaluation value, that is, the peak position 403 in FIG. 4corresponds to the just-focused position, a control corresponding to thefocused position is transmitted. After the completion of the adjustment,the system is shifted to the step 209 to terminate the autofocusadjustment process.

Embodiment 2

FIG. 7 shows a flow chart of the embodiment 2. The embodiment 2 shows anexample which includes the contents of the embodiment 1 and whereincalculation contents can be set by a user. The embodiment 2 is differentfrom the embodiment 1 only in the operation of a step 701 shown in FIG.7, that is, in that the user sets the calculation contents immediatelybefore the system performs the autofocus adjustment so as to calculate acombined image on the basis of the set calculation contents. Thecalculation contents can be specified by the user in the form of suchcontents displayed by a GUI as shown in FIG. 6.

A display screen 601 shown in FIG. 6 is the GUI for setting acoefficient (i.e., image addition ratio) to each detector in theaforementioned equation (1). On the GUI, the user can uniquely set amixture ratio of respective images in the equation (1) to enhance afocus accuracy according to each image. In this example, the user canset the above ratio by adjusting slide bars 605, 606 and 607 with use ofthe mouse 121. However, the setting method is not limited only to theexample or means on the display screen, and the user may enter numeralvalues directly with use of such an input unit as the keyboard 120. Theuser also can specify an image to be mixed with use of buttons 602, 603and 604. In this example, detectors 1 and 2 are set to be used forcalculation, but the display and setting methods are not limited to thisexample. In place of individually setting the mixture ratios ofrespective images, the user may select one of a plurality of choices.For example, the GUI can be set so that clicking the mouse on an area610 causes a display window 611 to appear and, when the user previouslysets choices 612, 613 and 614 according to the purpose, the user canselect one of the choices according to the purpose. Selection of thechoice 612 causes a window 615 to appear. In actual calculation, theuser previously sets mixture ratios corresponding to the contents of therespective choices to use them. In this example, since the user caneasily select different choices for different purposes, the user, evenwhen he has a bad command of using the system, can easily set it. Thedisplaying and selecting methods of the display window 611 are notlimited to the shown example. The function of an automatic selectionsetting button 609 will be explained layer in connection with anembodiment 3.

In this connection, upon the above setting, when an exemplary image isdisplayed as a display 608 after the calculation, the user can easilyobserve the specimen. The image to be then displayed may be an imagebeing currently picked up or an image stored by the user. The imagestoring method is carried out, for example, by preparing a button forimage storage and storing a desired image at arbitrary timing. The imagepicking-up method, including previously preparing an image and using animage stored in other works, is not limited to the example in the aboveembodiment. Also contents to be displayed on the display screen are notspecifically restricted to the above contents example. Stepscorresponding to the step 702 and subsequent steps are similar to thestep 202 and subsequent steps in the embodiment 1, and detailedexplanation thereof is omitted.

Embodiment 3

FIG. 8 shows a flow chart of this embodiment 3. The drawing issubstantially the same as the embodiment 2, but different therefrom inan image combining system and a parameter selecting method. Thedifference from the embodiment 2 is a function of automatically settingdetectors for acquiring images. FIG. 9 is a diagram for explainingparameters used in a calculation method selecting process in theembodiment 3.

When the user selects the automatic detector selection setting button609 in FIG. 6, this causes such sets of parameters 903, 904 and 905 asshown by a functional diagram 901 in FIG. 9 to be stored in the computer118 each time that the autofocusing operation is executed. A step 810 isprovided to determine an equation and parameters prior to the imagecalculation as shown by a functional diagram 902 in FIG. 9. In thedetermining method, one of the equations having the highest frequency ofexecution and parameters are set on the basis of history data previouslystored. As a result, images effective for focus adjustment can bereliably combined and effective focus adjustment can be achieved. Othersteps 801 to 809 are similar to the steps 701 to 709 in the embodiment2, and detailed explanation thereof is omitted.

When an error from the fitting function is large and the evaluationvalue is not suitable for evaluation in the step 805, the system goes toa step 811 to execute evaluation in another calculation scheme. Whenthere is another calculation scheme not executed yet in the steps 903,904 and 905 in the step 811; the system again sets the scheme notexecuted yet and executes its evaluation in a step 810. When the imageevaluation is already executed and confirmed for all the calculationschemes, the system goes to the step 808, where the system performs noautofocus adjustment and terminates its operation in the step 809. Themethod of again setting the calculation schemes in the step 811 may beexecuted by sequentially selecting the schemes or also by settingpriorities therefor according to a scheme switching method (to beexplained later) and selecting the scheme.

The calculation scheme switching method is not limited to the aboveexample. The scheme switching method may be carried out based onprobability and statistics, for example, the calculation scheme isdetermined as a standard scheme without involving its judging operationif the same calculation scheme has been executed a predetermined numberof times. The determining operation may be carried out based oninformation about wafer including chip-in coordinate and chip positionand information about wafer processing steps and wafer inspectionapparatus. For example, with respect to a wafer after subjected to theoperation of a specific step, when an image combined under conditions ofthe parameters 903 is especially effective for a specific part within achip, autofocus adjustment can be efficiently obtained by setting theparameters 903 in its coordinated range from the beginning. As data as areference of the determination, data collected during observation of thewafer or data previously prepared may be used, or data about acombination thereof may be used. With regard to the history information,a function of erasing the history according to user's specification canbe set as such an erase button 617 as shown in FIG. 6.

When the user sets such operations, the operations can be set byproviding automatic setting items to the GUI for setting of thedetectors as one setting, for example, with use of the automaticselection setting button 609 already shown in FIG. 6. This is only anexample and the present invention is not limited to the specificexample. For example, for the entire autofocus adjustment or settingsother than the autofocus adjustment; automatic setting items can beprovided with use of, e.g., the choice 612 of FIG. 6, and itscalculation scheme can be automatically selected through theaforementioned operation in response to the selection. The setting itemsare not shown in the GUI. However, it is also possible to automaticallyselect any of the detectors when the buttons 602, 603 and 604 of the GUIare not manually selected.

Embodiment 4

FIG. 10 shows a flow chart of the present embodiment 4. FIG. 11 shows aGUI for setting the operational contents of the autofocus function. Thefocus adjustment is carried out according to a procedure of steps 1001to 1010. First of all, the system determines a detector to be imaged inthe step 1001. At this time, the user can select the detector from a GUIsuch as a display screen 1101 shown in FIG. 11. Although the format ofthe GUI is not specifically limited to the format of the display screen1101; such a method as to provide explanations shown in an area 1102 andthat allows the user to select one of detectors by clicking one ofbuttons 1103, 1104 and 1105, is convenient as the format of the GUI. Itis also desirable that the automatic detector selection (to be explainedin the following discussion of this embodiment) be previously set as oneof the choices at this time point. In FIG. 11, it corresponds to buttons1106 and 1107. In the next step 1002, the system picks up images by thedetector selected while shifting the focus position. In this step, a setof images having gradually varied focus states can be obtained. In thestep 1003, next, the system calculates evaluation values for evaluatingthe focus states.

The evaluation values are calculated based on differential values of theimages. A pattern edge or the like can be easily observed. Such agrayscale level change of an edge at the contour of an image in thefocused state, however, is larger than a level change at a grayscalelevel change at the contour in the blurred image. From thisconsideration, a sum of pixel values after the image is differentiatedis calculated and the sum is used as the evaluation value. Maximum oneof such evaluation values is considered to be equal to a focusedposition.

The evaluation value and the fitting function are as already explainedin FIGS. 4 and 5. Fitting operation is carried out using a prescribedfitting function, and an error between the function and the evaluationvalue becomes a certain guideline for determining whether or not theselected image is suitable for the autofocus adjustment. When the errordata has an error level not larger than a constant value, the systemgoes to the step 1007 to calculate a focus position with the same data.When the error level is unsuitable, the system goes to a step 1005. Whenthe system executes the operations of the steps 1001 to 1004 for all theimages of the detector but still fails to have a small error from thefitting function in the step 1005, the system goes to a step 1009,returns the states of the objective lens and so on to their initialstates prior to the start of the system without performing the focuscalculation. The system then proceeds to a step 1010 to terminate theautofocus adjustment process. This operation can also be realized byshifting the system to the step 1007 using data about the detectorhaving smallest one of so-far obtained errors for calculation. It is notnecessary to carry out the above operation for all the detectors. Whenthe system performs calculation for all the images of the detector, thesystem goes to a step 1006, changes the detector to another detector,and returns to the step 1002 to again acquire images.

The system calculates a peak position in the step 1007 and then proceedsto the step 1009. In the step 1009, the system sends a signal to theobjective lens controller 112 so that the focus is adjusted at a focusedposition. Since the peak position of the evaluation value graph, thatis, since the peak position 403 in FIG. 4 corresponds to thejust-focused position, the system sends a control corresponding to thisposition. After completing the adjustment, the system goes to the step1010 and terminates the autofocus adjustment process.

Embodiment 5

FIG. 12 shows a flow chart of this embodiment 5. This embodiment isbasically the same as the embodiment 4, but different therefrom in amethod of determining one of the detectors in the step 1201. Thedifference corresponds to a function of automatically setting thedetector to acquire images. When the user selects an automatic detectorselecting button 1106 in FIG. 11, this causes the number of the detectorthen used to be stored in the computer 118 each time that the systemexecutes the autofocus operation. When the system sets a detector in astep 1201, the system sets one of detectors having highest one offrequencies of execution based on its history data. As a result, thesystem can reliably select images effective for focus adjustment and canefficiently perform focus adjustment. Since other steps 1202 to 1206 and1208 to 1210 are the same as the steps 1002 to 1006 and 1008 to 1010 inFIG. 10 in the embodiment 4, detailed explanation thereof is omitted.

Detector changeover is not limited to the aforementioned example. Thedetector changeover can be realized based on probability and statistics,for example, the detector is not determined as a standard detector ifthe detector has been executed a predetermined number of times. Or thedetermination may be carried out on the basis of information on waferincluding chip-in coordinate and chip position and information on waferprocessing steps, wafer inspection apparatus, etc. For example, withregard to a wafer after subjected to the operation of a specific step,when the detector 1 is effective for a specific part within a chip, theautofocus adjustment process can be efficiently executed by setting thedetector 1 in its coordinate range from the beginning. As data to beused as the determination reference; data collected during observationof the wafer may be used, previously-prepared data may be used, or datacorresponding to a combination thereof may be used. For historyinformation, such a button 1108 as shown in FIG. 11 is provided so thata function of erasing the history can be previously set.

When the user sets such operation, as a setting example, such anautomatic selection button 1106 as shown in FIG. 11 is provided so thatautomatic setting items can be provided and set in the area 1102 of theGUI for setting of detectors. However, this is given as an example andthe present invention is not limited to the specific example. Forexample, the entire autofocus operation or other operation can be set sothat an automatic setting button 1111 is provided and one of detectorsis automatically selected in response to the selection of the detectorthrough the above operation as shown by a display 1109 in FIG. 11.Although not shown by a setting item in the GUI, such an item may be setso that, when any of the detectors in the area 1102 is not selected, allthe detectors are automatically selected.

Embodiment 6

FIG. 13 shows a flow chart of the present embodiment 6. The presentembodiment is different from the embodiments 4 and 5 in that all imagesare captured and evaluated at the same time. In a step 1302, a focusposition is varied to acquire images from all the detectors. This stepis similar to the step 1002 in FIG. 10 in the embodiment 4, except thatimages from all the detectors are acquired. In a step 1303, next,evaluation values for each detector are calculated and its graph iscreated. This step is also similar to the step 1003 in FIG. 10 in theembodiment 4, except that the evaluation value calculation is carriedout for each detector. An fitting error is calculated by the computer118 on the basis of this data for each detector in a step 1304, andprocessing operation is branched according to the error.

Details of the branching operation is shown in FIG. 14. The systemselects one or ones of fitting errors found in a step 1402 lying withina prescribed error range, on the basis of data from a group of themeasured detectors 1401. When the system selects two or more fittingerrors, the system selects smallest one of the detector errors as in astep 1403, and is used for the focus adjustment in a step 1405. As theselection determination reference, all the data within the prescribederror range can also be used and an average of the graphs can be set tobe used in peak calculation (step 1404). When only one piece of data ispresent within the prescribed error range, the system performs focusadjustment using the data. When there is not present even a single pieceof data within the range, the system goes to a step 1406 where thesystem performs operation when corresponding data is not present in thestep 1309 of FIG. 13. In this way, when images of a plurality ofdetectors can be selected at the same time, images optimum for focusadjustment can be selected in single operation by evaluating the imagesat the same time. Since the operations of steps 1307 to 1310 are similarto the steps 1007 to 1010 of FIG. 10 in the embodiment 4, detailedexplanation thereof is omitted.

In all the embodiments 4 to 6, when whether which one of the detectorsis executed or was executed before or after the autofocus execution isdisplayed on the display screen, the user can understand what operationnow carried out, which is convenient. FIG. 15 shows an example of thiscase. In a display screen 1501 of the example, an image of the electronmicroscope is displayed in an area 1502, and information about thedetector which executes the autofocus operation is expressed as asuperimposition 1503 within the area 1502. The location of theinformation to be displayed is not limited to the inside of the frame ofthe area 1502 but may be outside of the frame. For example, marks 1504corresponding to detectors D1, D2 and D3 may be provided so that, whenthe corresponding detector was used, the mark of the used detector islit. The means for expressing the detector information is given only asan example and the present invention is not limited to the aboveexample. As another display example, the corresponding detectors may becontinuously displayed as superimpositions 1601, 1602 and 1603 as shownin FIG. 16. This expression is specially understandable to the user whenthe detectors are stepwise changed as in the embodiments 4 and 5.

As has been explained in the foregoing, in accordance with the aboveembodiments, when an image capable of being picked up only by a specificdetector is included in images, stable autofocus adjustment can beattained by an electron microscope having a plurality of detectors whichcan pick up images from each detector while changing a focus position(Z) and evaluating part of the images or ones of the images stepwise orat the same time. Thus, the electron microscope can detect a focusedposition even when autofocus adjustment using a single image isdifficult.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A scanning electron microscope comprising: an electron lens forconverging an electron beam emitted from an electron source onto aspecimen; a pair of first and second detectors and a third detector fordetecting secondary signals from said specimen when irradiated with saidelectron beam, said first and second detectors being provided to bespaced from each other by a predetermined spacing; a storage unit forgenerating and storing first, second and third pieces of secondarysignal data from the secondary signals detected by said first, secondand third detectors at respective focuses, said focuses of said electronbeam to said specimen are changed a plurality of times; a datacalculation unit for performing combining operation to combine saidfirst, second and third pieces of secondary signal data at therespective focuses; and a focus adjustment unit for calculating anevaluation value based on an intensity signal obtained by subjectingsaid combined secondary signal data at the focuses to predetermined dataconversion, and adjusting a focus of said electron beam to said specimenusing the focuses calculated based on said evaluation values for thefocuses, wherein: said combining operation is carried out by combiningsaid first, second and third pieces of secondary signal data at thefocus of said electron beam according to a preset equation, and saidequation is an equation expressed by αSE+βL+γR, wherein α, β and γdenote coefficients respectively, and L, R and SE denote first, secondand third secondary signal data respectively.
 2. A scanning electronmicroscope according to claim 1, wherein said coefficients α, β and γcan be specified with use of an input unit.
 3. A scanning electronmicroscope comprising: an electron lens for converging an electron beamemitted from an electron source onto a specimen; a pair of first andsecond detectors and a third detector for detecting secondary signalsfrom said specimen when irradiated with said electron beam, said firstand second detectors being provided to be spaced from each other by apredetermined spacing; a storage unit for generating and storing first,second and third pieces of secondary signal data from the secondarysignals detected by said first, second and third detectors at respectivefocuses, said focuses of said electron beam to said specimen are changeda plurality of times; a data calculation unit for performing combiningoperation to combine said first, second and third pieces of secondarysignal data at the respective focuses; and a focus adjustment unit forcalculating an evaluation value based on an intensity signal obtained bysubjecting said combined secondary signal data at the focuses topredetermined data conversion, and adjusting a focus of said electronbeam to said specimen using the focuses calculated based on saidevaluation values for the focuses, wherein said combining operation iscarried out by adding differences between said first secondary signaldata and said second secondary signal data corresponding to therespective focuses to said third secondary signal data at the focuses ata predetermined ratio.
 4. A scanning electron microscope comprising: anelectron lens for converging an electron beam emitted from an electronsource onto a specimen; a pair of first and second detectors and a thirddetector for detecting secondary signals from said specimen whenirradiated with said electron beam, said first and second detectorsbeing provided to be spaced from each other by a predetermined spacing;a storage unit for generating and storing first, second and third piecesof secondary signal data from the secondary signals detected by saidfirst, second and third detectors at respective focuses, said focuses ofsaid electron beam to said specimen are changed a plurality of times; adata calculation unit for performing combining operation to combine saidfirst, second and third pieces of secondary signal data at therespective focuses; and a focus adjustment unit for calculating anevaluation value based on an intensity signal obtained by subjectingsaid combined secondary signal data at the focuses to predetermined dataconversion, and adjusting a focus of said electron beam to said specimenusing the focuses calculated based on said evaluation values for thefocuses, wherein said focus adjustment unit applies differentiatingoperation to said combined secondary signal data at the respectivefocuses to calculate pixel values, calculates an evaluation value basedon a sum of the pixel values, and adjusts the focus of said electronbeam to said specimen using the focus calculated based on maximum one ofthe evaluation values.
 5. A scanning electron microscope according toclaim 4, wherein said focus adjustment unit controls said electron lenson the basis of the focus calculated based on said maximum evaluationvalue.
 6. A scanning electron microscope comprising: an electron lensfor converging an electron beam emitted from an electron source onto aspecimen; a pair of first and second detectors and a third detector fordetecting secondary signals from said specimen when irradiated with saidelectron beam, said first and second detectors being provided to bespaced from each other by a predetermined spacing; a storage unit forgenerating and storing first, second and third pieces of secondarysignal data from the secondary signals detected by said first, secondand third detectors at respective focuses, said focuses of said electronbeam to said specimen are changed a plurality of times; a datacalculation unit for performing combining operation to combine saidfirst, second and third pieces of secondary signal data at therespective focuses; and a focus adjustment unit for calculating anevaluation value based on an intensity signal obtained by subjectingsaid combined secondary signal data at the focuses to predetermined dataconversion, and adjusting a focus of said electron beam to said specimenusing the focuses calculated based on said evaluation values for thefocuses, wherein said focus adjustment unit applies differentiatingoperation to the combined secondary signal data at the focuses tocalculate pixel values, calculates an evaluation value based on a sum ofthe pixel values, determines a fitting function followed by theevaluation values, and adjusts the focus of said electron beam to saidspecimen using a focus providing a peak value to the fitting function.7. A scanning electron microscope according to claim 6, wherein saidfocus adjustment unit calculates an error to said fitting function, andwhen the error is in a predetermined reference error range, adjusts thefocus of said electron beam to said specimen using a focus providing thepeak value of said fitting function.
 8. A scanning electron microscopeaccording to claim 7, wherein, when said error is out of the referenceerror range, the focus of said electron beam to said specimen ismaintained.
 9. A scanning electron microscope comprising: an electronlens for converging an electron beam emitted from a electron source ontoa specimen; a pair of first and second detectors and a third detectorfor detecting secondary signals from said specimen when irradiated withsaid electron beam, said first and second detectors being provided to bespaced from each other by a predetermined distance; a storage unit forgenerating and storing first, second and third secondary signal datafrom the secondary signals detected by said first, second and thirddetectors at respective focuses, and said focuses of said electron beamto said specimen are changed a plurality of times; a data calculationunit for calculating said first, second and third secondary signal dataat the respective focuses; an input unit for externally receiving datanecessary for the calculating operation of said data calculation unit;an image display unit for displaying a command menu and indicating inputdata from said input unit as a result of said calculating operationexecuted by said data calculation unit and indicating contents of saidcalculating operation executed by said data calculation unit; a focusadjustment unit for calculating an evaluation value on the basis of anintensity signal obtained by applying predetermined conversion tosecondary signal data at the focuses executed and calculated by saiddata calculation unit, and adjusts the focus of said electron beam tosaid specimen using the focuses based on said evaluation valuescorresponding to the focuses, wherein: said calculating operation ofsaid data calculation unit is carried out by combining said first,second and third secondary signal data at the focus of said electronbeam according to a preset equation, and data for modifying coefficientspreset in said equation are input from said input unit.
 10. A scanningelectron microscope according to claim 9, wherein a storage unit forstoring the coefficients of said equation is provided, and, when thecoefficients are modified, the modified coefficients are added to saidstorage unit.
 11. A scanning electron microscope according to claim 10,wherein one of said coefficients stored in said storage unit having ahigh frequency of use in the focus adjustment of said electron beam isselected from the coefficients.
 12. A scanning electron microscopeaccording to claim 11, wherein said coefficients stored in said storageunit and said frequencies are erased from said input unit.
 13. Ascanning electron microscope comprising: an electron lens for convergingan electron beam emitted from an electron source onto a specimen; a pairof first and second detectors and a third detector for detectingsecondary signals from said specimen when irradiated with said electronbeam, said first, second and third detectors being provided to be spacedin each pair from each other by a predetermined spacing; a storage unitfor generating and storing first, second and third pieces of secondarysignal data from the secondary signals detected by said first, secondand third detectors for respective focuses of said electron beam to saidspecimen changed by a plurality of times; a data calculation unit forcalculating said first, second and third pieces of secondary signal dataat the respective focuses; a detector selection unit for detecting atleast any one of said first, second and third detectors; and a focusadjustment unit for generating secondary signal data from the detectorselected by said detector selection unit by changing the focus of saidelectron beam, calculating an evaluation value of the secondary signaldata for the respective focuses changed based on intensity signalsobtained by subjecting the generated secondary signal data topredetermined data conversion, determining a fitting functioncorresponding to the evaluation value, calculating an error between theevaluation value and said fitting function, adjusting the focus of saidelectron beam to said specimen using a focus giving a peak value to saidfitting function when the error is in a predetermined reference range,selecting the detector different from the detector selected by thedetector selection unit when the error is out of the predeterminedreference range, calculating an error between the evaluation value andthe fitting function with respect to the different detector, andadjusting the focus of said electron beam to said specimen using focuseshaving the errors lying in the predetermined reference range.
 14. Ascanning electron microscope according to claim 13, wherein said focusadjustment unit maintains the focus of said electron beam to saidspecimen when said error is out of the predetermined reference rangeeven after repetitive selection of said detectors by said detectorselection unit.
 15. A scanning electron microscope according to claim13, wherein said detector selection unit selects all of said first,second and third detectors, and said focus adjustment unit adjusts thefocus of said electron beam to said specimen using a focus correspondingto minimum one of errors between said evaluation values calculated forthe respective selected detectors and said fitting function lying in thepredetermined reference range.
 16. A scanning electron microscopeaccording to claim 13, wherein said detector selection unit selects allof said first, second and third detectors; and said focus adjustmentunit calculates an average value of said evaluation values for thedetectors having errors lying in the predetermined reference range outof errors between said evaluation values calculated for the respectiveselected detectors and said fitting function, and adjusts the focus ofsaid electron beam to said specimen using a focus providing a peak valueto said fitting function corresponding to said average evaluation value.17. A scanning electron microscope according to claim 13, wherein astorage unit for storing a selection history of the detector selected bysaid detector selection unit is provided, and detector selectingoperation of said detector selection unit is executed on the basis ofthe selection history.
 18. A scanning electron microscope according toclaim 13, wherein detector selecting operation of said detectorselection unit is executed based on probability or statistics.
 19. Ascanning electron microscope according to claim 13, wherein saiddetector selection unit selects one of the detectors according to a stepof manufacturing an inspection target.